Seven Plutella xylostella (L.) populations were compared for resistance to Bacillus thuringiensis subsp. kurstaki Berliner in leaf-dip and diet-incorporated bioassays. The persistence of the microbial insecticide incorporated in diet was studied at 2 storage temperatures (26.7 and 5°C). The effect of larval age on toxicity of a discriminating concentration of Javelin incorporated in diet (20.5 mg [Al]/liter) was also studied. In the leaf-dip bioassay, LC50s of 6 field populations were 2.6- to ≈829-fold higher than the LC50 of Geneva 88, the susceptible population, whereas LC50s in the diet-incorporated bioassay were 1.7- to ≈42-fold higher in the field populations compared with the LC50 of Geneva 88. Despite the difference in resistance ratios between bioassay methods, LC50s were significantly correlated. Both methods were effective in distinguishing between susceptible and resistant populations, With 1 exception, slopes of the regressions in individual populations were not significantly different. We concluded that both bioassay techniques were suitable for monitoring evolution of resistance to B. thuringiensis in field populations of P. xylostella. Compared with diet prepared the day of the tests, the toxicity of B. thuringiensis did not vary significantly in diet stored for 10 d at 26.7°C, and the LC50s of Javelin did not vary significantly in diet stored for 14 d at 5°C. Mortality of 2nd and 3rd instars was not significantly different when exposed to a discriminating concentration of Javelin incorporated in diet, but mortality was significantly lower in 4th instars. The diet-incorporated B. thuringiensis assay can be used as a resistance test kit for resistance studies.
No takes yet. Share an insight, caveat, or question.
Pérez et al. (1997) studied this question.